In general, an airstream around a vehicle body during traveling of a vehicle may be separated (may be away from the vehicle body) or disturbed according to the vehicle body structure. If a mainstream (the strongest and fastest flow) of the airstream is far away from the vehicle body (if the boundary layer is thickened), the aerodynamic force (air restraining force) with respect to a vehicle is decreased. If the space in which the aerodynamic force is weak is increased, the vehicle becomes unstable. In Patent Literature 1, it is disclosed that a flow correction fin is provided on the lower portion of a vehicle body, the flow of fluid at the lower portion of a vehicle is corrected due to the flow correction fin and speed of the flow is increased, and stability of the vehicle is improved. In addition, in Patent Literature 5, it is disclosed that a convex bump is provided at the rear end portion of the roof of a vehicle, and thereby, air resistance of a vehicle body is decreased.